Medical imaging technology has made remarkable advances in recent years, including developments and improvements in computed tomography (“CT”), magnetic resonance imaging (“MRI”), functional magnetic resonance imaging (“fMRI”), single photon emission computed tomography (“SPECT”), and positron emission tomography (“PET”).
PET imaging has revolutionized imaging of internal biological regions by providing functional images of a patient or other region of interest. Positron emission tomography is a nuclear medicine medical imaging technique that produces a three-dimensional image or map of functional processes in the body, e.g., imaging that illuminates chemical and metabolic activity in the patient. The role of PET imaging in oncology research and patient care, in particular, is growing due to the ability of PET to add unique functional information to that obtained by conventional anatomical imaging modalities, for example CT.
PET scanning is an emissive technique wherein a short-lived radioactive tracer isotope, chemically combined with a metabolically active molecule such as a sugar, is injected into the subject. The metabolically active molecule becomes concentrated in the tissues of interest, concentrating the tracer isotope in regions of such activity. After injecting the isotope, the patient is placed on the scanner. As the injected isotope decays it emits a positron that annihilates with an electron, producing a pair of gamma rays or photons that travel in opposite directions. In general terms, the emitted photons are detected when they reach a scintillator material in the scanning device, creating a burst of light that is detected by photomultiplier tubes.
The detection technique relies on the coincident detection of the pair of photons to identify valid signals. Photons that are not detected within a few nanoseconds of each other are ignored. A straight line through the locations in the detector where the coincident photons are detected is called the line of response (“LOR”). The location of the positron emission is therefore known to lie somewhere along the LOR. The PET scanner uses the pair detection events and the LORs to map the density of the tracer isotope within the body. In a typical system, the images are generated along parallel slices separated by about 5 mm and the images are then combined to produce a three-dimensional image or model of the region of interest. The resulting map shows where the tracer isotope has become concentrated, identifying regions of metabolic activity in the body.
A dose calibrator (radioisotope calibrator) is a device used in nuclear medicine that measures the total energy of a specific radionuclide in units of Curies (Ci), millicuries (mCi), or microcuries (μCi). It includes a hollow, lead-shielded cylinder, into which radionuclides are lowered for measurement. Such devices can be programmed for specific radioisotopes, or adjusted for isotopes not preprogrammed. A dose calibrator is commonly used to obtain measurements of the total radioactivity of isotopes prior to administration to patients undergoing nuclear medicine diagnostic imaging procedures or radioisotope therapy procedures. Regulatory authorities specify when a radioisotope dose calibrator will be used and the timing of required quality control checks (constancy, accuracy, linearity, and geometrical dependence).
Currently, a dose calibration source standard is used for calibration of the dose calibrator. The calibration source includes a cylindrical vial comprising a predetermined amount of the radionuclide to be calibrated, together with a decay calendar, which allows a user to determine the amount of radionuclide present in the source at the time of testing. The current dose calibration source standards are designed to closely approximate the geometry of unit dose radiopharmaceuticals dispensed in vials by radiopharmacies and may therefore only partially meet the standards implied in CEI-IEC 61145 “Calibration and Usage of Ionization Chamber Systems for Assay of Radionuclides;” CEI-IEC 1303 “Medical Electrical Equipment—Radionuclide Calibrators—Particular Method of Describing Performance;” ANSI N42.13-1986 “Calibration and Usage of ‘Dose Calibrator’ Ionization Chambers for the Assay of Radionuclides;” and 10 CFR 35.50 “Possession, Use, Calibration, and Check of Dose Calibrators.”
Nuclear medicine practitioners generally administer radiopharmaceuticals using a syringe. The practitioner places the syringe containing the radiopharmaceutical into the pre-calibrated dose calibrator to assay its content. The syringe has a different body shape from that of the calibration vials used as calibration standards and, due to the shape of the syringe and configuration of the dose calibrator, is positioned in the dose calibrator with the syringe body in a different location from the vial.